Finnie said: "Woodland is good for people, for wild plant and animal species, and for the rural economy. The Western Isles are one of the most deforested parts of Scotland, so it makes sense to use already existing powers to defend the remaining trees."
The 2004 Native Woodlands Habitat Action Plan estimated that there was just 200 hectares of semi-natural woodland left on the islands, less than one-thousandth of their total area, and set a target of ensuring no net loss in area or reduction in quality of these ecologically and historically precious areas, he pointed out.
"The rules for TPOs specify that the Order must protect either 'amenity' or trees of 'cultural or historical significance'. Any further loss of trees on the islands would clearly be a blow to local people's wellbeing – or 'amenity' – and there can be no doubt that our remaining native woodland is of both cultural and historical significance."
Finnie is Scottish Environment LINK's Species Champion for the alder, a tree that is essential to reforestation because it can grow in marshy, unforested areas and improves the soil allowing other tree species like willow to take hold.
The council had earlier stated that such a move would go beyond the statutory role of TPOs.
But in a new letter to council chief executive Malcolm Burr, Finnie wrote: "I disagree that it is self-evident that a TPO protecting all of the Western Isles' non-commercial trees would fail the tests set by statute.
"Given the precarious situation of woodland in the Western Isles, and the great importance of woodland to environmental and public health, I would be grateful if you would reconsider my proposal to make a Tree Preservation Order, or a number of contiguous Tree Preservation Orders, to defend the Western Isles' non-commercial trees."
A former policeman and SNP then independent MSP, Finnie was elected as Scottish Green Party regional member the Highlands and Islands region earlier this year.